1. Field of the Invention
The present invention mainly relates to a power semiconductor device and, more particularly, to a semiconductor device which can prevent arc generation, damage of a case, and the like when a failure occurs.
2. Description of the Related Art
The power semiconductor device includes a device in which one element is mounted on one semiconductor substrate, and a device in which a plurality of elements are mounted on one semiconductor substrate. A prior art power semiconductor device will be described hereinafter, exemplifying a power n-p-n transistor which belongs to the former type of device.
FIG. 1 is a typical sectional view of the conventional transistor. An emitter layer 2 and a base layer 3 are formed on one surface of a semiconductor substrate 1. An Al emitter electrode 4 and an Al base electrode 5 are formed on the surfaces of the layers 2 and 3, respectively. An n-type collector layer 6 is formed on the other surface of the substrate 1, and the collector is soldered to a Cu base 7.
A metallic emitter terminal 11E consisting of an emitter terminal block 9E and an external emitter terminal 10E connected to the terminal block 9E is formed on the Cu base 7 through an insulating material 8E. Similarly, a metallic base terminal 11B consisting of a base terminal block 9B and an external base terminal 10B connected to the terminal block 9B is formed on the Cu base 7 through an insulating material 8B.
The emitter electrode 4 and the emitter terminal block 9E, and the base electrode 5 and the base terminal block 9B are connected by Al wires 12 and 13, respectively, by means of ultrasonic welding. The entire transistor is covered by a plastic case 14, and an internal space 15 of the case is filled with a resin. In this transistor, the emitter electrode 4 is connected to the metallic terminal 11E via the metallic wire 12. When an excessive current flows upon failure, the wire 12 is fused. Alternatively, when an excessive current flows upon failure, a bonding portion between the metallic wire and the electrode on the semiconductor substrate, or between the metallic wire and the metallic terminal is separated. If a voltage of 100 V or more is applied to this portion, an arc is generated and the metallic wire is easily fused.
When the metallic wire is fused due to flow of an excessive current, and a voltage of 100 V or more is applied to the fused portion, an arc is generated. The temperature in the case is extremely increased and the case is scattered. Not only this semiconductor device, but also circuit portions around the device may be damaged.
In order to prevent the above problem, an attempt has been made to increase the cross-sectional area of the metallic wire as much as possible. However, the thickness of the wire which allows reliable connection by ultrasonic-welding is limited. More specifically, in the case of an Al wire, its diameter can be increased only within 500 .mu.m.
It may be proposed to connect a thicker wire by, e.g., soldering. However, in the case of an A.+-.electrode, it is difficult to perform soldering.
Even if a plurality of metallic wires are connected in parallel, a current capacity for an excessive current upon failure is not always increased in proportion to the number of wires, and the current capacity is limited. In addition, an improved high power semiconductor device wherein semiconductor chips are coupled to terminals via bonding wires is disclosed in U.S. Pat. No. 4,518,982, as another prior art.
The power transistor has been described above as a prior art. However, even if another power semiconductor device in which a metallic terminal is connected to a metallic electrode via a wire, and an excessive current exceeding a fusing current flows through the metallic wire upon failure is used, the above-described phenomena, i.e., arc generation and damage of the case occur upon failure.
In view of this problem, a semiconductor rectifier which has a p-n junction is fastened with its connecting component to one side of a common metallic base plate is described in U.S. Pat. No. 4,047,197 as a prior art. In this prior art, a metallic wire is not used and the above disadvantages of the metallic wire can be avoided.
The connection between the metallic base plate and the connecting component, however, allows formation of a current path. Therefore, the step of soldering or the like is provided in order to achieve reliable electrical contact. For this reason, processing precision is degraded, and a range of metal selection is limited.